The invention is concerned with the general style of inductor commonly used on layered supportive media, such as printed circuit boards (PCBs), semiconductor chips, and integrated circuits (ICs). These types of inductors are often referred to as “spiral inductors.” Spiral inductors, and resonant circuits containing spiral inductors, are used, for example, in radio frequency and microwave (RF/MW) circuits, including, but not limited to, filters, tuned circuits, matching circuits, RFD devices, and biomedical devices.
Often, a spiral inductor is part of an inductive-capacitive LC circuit. Frequently, such LC circuits are used as building blocks for larger circuits. In order to build an LC circuit, it is common for the circuit designer to add capacitance to the LC circuit in a region near the spiral inductor in order to augment the self-capacitance associated with the spiral inductor itself. This added capacitance can take up valuable area in the circuit layout. Frequently, circuit designers look for methods of reducing layout area and reducing the number of components when designing electrical circuits.
A spiral inductor can exhibit higher-than-desired energy loss, depositing energy within its conductive trace and in matter surrounding the trace. Energy loss is often quantified in terms of a unitless quality factor (Q). Circuit designers frequently aim to reduce energy loss in circuits.
When a spiral inductor is used in an electrical circuit, energy losses occur as a result of interaction of the physical circuit with the spiral inductor's E- and B-fields. This loss is due to: (1) currents and resistance in the spiral inductor's own conductive trace; and (2) the interaction of the inductor's E- and B-fields with matter external to the conductive trace of the inductor.
Consider a spiral inductor 101 having more than one turn. As the conductive trace that defines the spiral inductor approaches the central region 104 of a spiral pattern, an underpass 105 (or an overpass) is usually utilized to bring the inductor's inside lead 107 (its inner electrical terminal) to the outside of the spiral's outer perimeter 102. The connective feature of an underpass 105 (or an overpass) is usually necessary for the circuit, and complicates to some degree the layout of spiral inductor circuits. This complication is partly because a second conductive layer is needed, together with electrical connections that extend to the trace of the underpass (or overpass) layer from the trace of the spiral inductor.
The LC circuits and subcircuits based on this invention do not need to operate at their resonance frequency. Inductive-capacitive circuits can also operate at frequencies above or below their resonance frequencies.
A spiral inductor, having a first terminal 106 (or first “lead”) and a second terminal 107 (or second “lead”), is laid out on an insulating substrate either by etching or otherwise forming a conductive trace in a spiral pattern, which is known in the art of electrical circuit design. As the layout of the spiral's trace 101 (conductive path) winds toward the central region 104 of the spiral pattern, an underpass 105 (or an overpass) is utilized to bring the inductor's inner lead (inner terminal) to the outside 102 of the spiral from the inner region of the spiral 104, to permit easier connection to an electrical circuit. This underpass 105 (or overpass) is connected to the primary spiral with vertically oriented conductive segments or “vias.” The use of vias is a general technique that is known in the art of circuit design. Sometimes, the spiral inductor stands alone, and is used alone, without direct conductive electrical connection to a circuit. In this case, electrical energy is coupled into the primary spiral inductor and out of the spiral inductor electrically and magnetically.
A circuit's energy efficiency affects crucial performance parameters such as power consumption, electrical heating, random noise, phase noise, and circuit sensitivity. A useful figure-of-merit when characterizing an LC circuit is the tank-Q or “QTANK.” The unitless value of tank-Q quantifies electrical energy loss. In lower-Q circuits, a greater amount of energy is lost compared to higher-Q circuits due from the interaction of the inductor's E- and B-fields with itself and with surrounding matter. Some types of matter interact more substantially with the E- and B-fields. In particular, some materials, including electrical conductors, dielectrics with nonzero loss tangents, and semiconductors, can interact in an electrically parasitic manner with an inductor's electric and magnetic (E- and B-) fields. These interactions result in circuit energy that is deposited as heat energy. Thus, a higher value for the Q-factor indicates, in general, better circuit performance and less electrical heating.
For a spiral inductor circuit, particularly for those that form of inductor-capacitor (LC) circuits, the total layout area is another important figure-of-merit. The circuit designer generally prefers compact layouts in order to conserve valuable circuit area and to reduce parasitic effects in circuits. In the art of electrical design, LC circuits and LC subcircuits (i.e., smaller circuits that are sometimes used as building blocks) are commonly achieved by laying out parallel plate capacitance 708 near the inductor's two terminals or by attaching discrete capacitors across the inductor's two terminals, or in series with either or both of the inductor's two terminals.
To improve the Q-factor for a spiral inductor's inductance-capacitance circuit, techniques have been developed in the circuit design arts to help shield, terminate, or redirect the spiral inductor's E-fields away from the surrounding higher-loss matter. Usually, in the art, this involves designing a conductive pattern for placement under the spiral inductor in the region between the primary spiral inductor and the lossy media. (The conductive pattern may also be placed above the spiral inductor.) The conductive pattern is comprised of metal or other conductor and is sometimes referred to as a “plate” or “shield.” For example, in the art, these conductive trace patterns are placed in an intermediate layer or in intermediate layers, below the bottom surface of the spiral inductor and above nearby lossy materials, on a separate conductive layer or layers. (A high-loss or “lossy” dielectric material is a dielectric with a non-zero loss tangent.) In the art, these conductive plates typically have patterns cut into them, forming strips of conductor with non-conducting gaps. In the art, shields fabricated from a fully continuous layer of metal—without specific patterns cut or etched into it—have also been used for shielding the inductor's E-fields.
In circuits, a spiral inductor's energy loss can result in heating, and this heating can exacerbate the circuit's unwanted energy loss further because most materials have a positive temperature coefficient of resistance. Shields in the form of patterned conductive plates have been devised in the art to lessen this energy loss, but often have limitations of their own, including limited Q-factor values, consumption of layout area, necessary ground connections, or insufficient capacitance (in cases where capacitance is desired)